Theodore, who is described (as a "stringy Slytherin boy") but not named, and Harry are in the same section of Hagrid's Care of Magical Creatures class on the day after Hagrid's return from his mission, when Hagrid chooses to teach Thestrals. Harry sees them, and notes that most of the students are staring blankly around, just as Ron is. The two other students who seem able to see the Thestrals are Nott and Neville Longbottom.

After Harry's interview appears in the Quibbler, Harry sees "a weedy-looking boy... called Theodore Nott" in the library, talking with Draco Malfoy, Crabbe, and Goyle. Though they are apparently talking about Harry's interview, which named all their fathers as Death Eaters returning to Lord Voldemort's side, they cannot admit to having read it, because possession of the Quibbler is an expulsion offence.

Theodore was mentioned by Blaise Zabini as one of the students who Professor Slughorn plans to recruit for his Slug Club. Ultimately Nott was not invited due to his father being recently arrested after being discovered as a Death Eater, despite the elder Nott apparently being one of Slughorn's favorite students.

Very few people have progressed to NEWT-level studies in Slughorn's Potions class: Harry, Ron, and Hermione alone among the Gryffindors, Ernie Macmillan from Hufflepuff, four Ravenclaw students including Terry Boot and Michael Corner, and four from Slytherin house including Draco Malfoy and Theodore Nott. Nott here is often exchanging asides with Draco, as Draco's henchmen, Crabbe and Goyle, did not do well enough in their Potions O.W.L. to continue on in that course, even given Slughorn's slightly lower entrance requirement.

Theodore originally had a large scene in which he and Draco Malfoy discussed Harry while their respective fathers, Nott and Lucius Malfoy, discussed Death Eater business off camera. The author mentions that while she tried to fit the scene into two different books, it didn't work well either place and so was laid to rest.

Theodore Nott was one of the students to have checked out the library copy of Quidditch Through the Ages, a book mentioned in the story and reproduced by the author.

Nott is described as a clever boy. Unlike most of the other Slytherins in Harry's year he doesn't seem to take part in bullying or otherwise show signs of aggression. It's also likely that he is talented at least in the field of potions due to attending the class at NEWT level.

Theodore seems to be a friend of Draco's though he does not usually hang out with the other members of Malfoy's gang. Despite this friendship Nott is apparently a loner who usually keeps to himself and doesn't feel the need to join gangs.

He is the son of the Death Eater Nott who appears at Voldemort's rebirth and later as one of the Death Eaters who Lucius brings with him to the Ministry.

Some fans have noticed a lot of similarities between Nott and Severus Snape when the latter was a student. For starters, both were Slytherins who were associated with gangs of Death Eater supporters despite generally preferring to be alone. Both are also described as being unusually clever and (if Nott's attendance in the NEWT level potions class is any indication) skilled at potion making. They are also both described as being weakly built and have frequent comparisons to animals (Snape with bats, Nott with rabbits). It's unknown if these similarities were intentional and if they give insight on Nott's possible character.

Intermediate warning: Details follow which you may not wish to read at your current level.